This invention relates generally to systems e.g., radio frequency identification (RFID), for identifying the presence or absence of members of a defined member set. More particularly, the invention is directed to a system which includes multiple active senders and a central monitor, with each sender being configured to repeatedly transmit a signal representing a uniquely encoded identification frame. A composite signal received by the monitor is decoded to separately identify each of the transmitting senders.
Various RFID systems are described in the patent and technical literature which are used to identify the presence of specially configured identification tags within a prescribed zone. Depending upon the system application, the tags can be affixed to a set of inanimate objects, or animals, or humans. Prior literature describes both passive tags which require interrogation and active tags which are powered and can initiate an identification signal transmission.
The present invention is directed to a system which allows multiple senders to asynchronously transmit identification codes via a common communication channel (e.g., RF) to enable a central monitor to identify the presence (or absence) of each sender within the monitor""s detection zone. Each sender is configured to repeatedly transmit a uniquely encoded identification frame. A frame, in accordance with the invention, is comprised of pulses spaced to minimize pulse collisions and configured to tolerate occasional collisions without impairing the monitor""s ability to separately identify each transmitting sender.
Systems in accordance with the invention are useful in a wide variety of applications for monitoring the presence (or absence) of a member of a defined set of members. For example, the teachings of the invention can be used in a warehouse system to log the presence of items in inventory or in a man-overboard (MOB) system for monitoring the absence of persons on a recreational boat. The indication of presence or absence can be used to either directly actuate an output device, e.g., an alarm, or can be logged for further processing by a host computer. In such systems, each member of the set (e.g., each inventory item or each person on the boat) carries a sender.
Senders, in accordance with preferred embodiments of the invention, comprise small inexpensive electronic devices which can be carried by, attached to, or worn by the members of the set being monitored. Each sender is preferably self powered, typically by a battery, and is capable of periodically transmitting, low power, short duration pulses over a long life which can, in some applications, be measured in years. A power level is selected to support a communication range appropriate to the application; e.g., from less than one hundred to several hundred feet.
In accordance with the invention, each sender is configured to repeatedly transmit a unique identification frame characterized by a pulse pattern comprised of active intervals spaced by inactive (or xe2x80x9cquietxe2x80x9d) intervals. The inactive intervals have variable length durations which are preferably pseudorandomly selected so that each sender defines a unique sequence of inactive interval durations.
Active intervals in accordance with the invention are comprised of one or more pulse intervals. That is, in a first embodiment, each active interval can comprise a coded burst of multiple pulses. In a different embodiment, each active interval comprises a single pulse. A quiet interval is defined between successive active intervals.
In accordance with certain preferred embodiments of the invention, the durations of quiet intervals within a frame are used to define the identification code (i.e., ID code) of each sender. As an example, an identification frame format can comprise six active intervals bounding five quiet intervals. The duration of each quiet interval can, for example, have a resolution of between 1 and 64 clock periods. These exemplary quantities would provide for a pool of 645 possible ID codes. A different ID code is assigned to each of the multiple senders. A typical clock period can equal 100 microseconds, for example, so that an identification frame using the aforementioned exemplary numbers could have a duration of up to 32.6 milliseconds [i.e., (6 active intervals*100 microseconds)+(5 quiet intervals * 64*100 microseconds)]. In such an exemplary embodiment, the pulse duty cycle of each sender is extremely low, i.e., the sender transmits for only 600 microseconds out of each identification frame duration of 32.6 milliseconds. In certain applications of the invention, sender transmission cycles can be comprised of an identification frame and a silent period. This further reduces the duty cycle and power consumption of each sender.
Moreover, the number of active senders sharing a common channel in a typical application of the invention is several orders of magnitude less then the number of possible ID codes. All of the foregoing factors contribute to minimize the occurrence of pulse collisions on the channel and enable a central monitor to interpret a received composite signal to reliably and unambiguously identify each transmitting sender.
More particularly, embodiments of the invention are characterized by sender pulse patterns which are inherently sparse, i.e., low density, and inherently redundant. Accordingly, embodiments of the invention are able to tolerate pulse collisions by effectively treating colliding pulses as belonging to every sender that might otherwise be identified by other pulses in the received composite signal. Because of the very low average transmission time of each sender and the inherent tolerance to pulse collisions, embodiments of the invention are able to accommodate a large number of asynchronously transmitting senders.
A central monitor in accordance with the invention includes a memory for storing a repertoire of sender ID codes. This repertoire can be stored or acquired in several different ways; e.g., the ID codes can be 1) preprogrammed into nonvolatile memory at the factory, 2) uploaded from a host processor, 3) programmed into monitor memory via a manual agent such as a keypad or switches, 4) learned by the monitor during an installation or set-up procedure, 5) learned by direct electrical contact or by an auxiliary channel such as an infrared (IR) light emitting diode (LED), 6) learned by repeated recognition. Regardless of how the ID code repertoire is acquired, the monitor operates to compare each ID code stored in its repertoire against a received composite signal history accumulated by the monitor. More particularly, the monitor includes a memory which preferably accumulates a history of the composite signal pulse pattern over a time equivalent of at least the longest ID frame. The history is updated at a rate to reflect the received pulses and intermediate interval durations. Each time a new pulse is received, the monitor is able to compare the stored ID codes with the history to identify a match.
In certain applications of the invention, e.g., a man-overboard system, each sender is expected to xe2x80x9ccheck-inxe2x80x9d within a certain xe2x80x9ctime-outxe2x80x9dperiod, e.g., every three seconds. If a sender""s ID code fails to appear in the received composite signal within the time-out period, then the monitor generates an alarm. In systems intended for other applications, the monitor can respond more quickly or more slowly or can merely maintain a log of the presence or absence of a sender. In still other systems, the monitor operates to report sender check-ins to a host computer.
In accordance with a first exemplary embodiment (serial code burst) of the invention, each sender transmits a coded burst of pulses during each frame active interval. Each pulse burst defines an ID code uniquely identifying the sender. The bursts are spaced by quiet intervals whose durations are preferably pseudorandomly selected. Thus each sender will transmit its bursts based on a different sequence of quiet interval durations so that successive burst collisions are highly improbable.
In accordance with a second exemplary embodiment (code burst/pulse) of the invention, a coded pulse burst defining a sender""s ID code is transmitted once per frame and marker pulses are additionally transmitted during each frame to bound the quiet intervals. In this embodiment, the sequence of quiet interval durations can redundantly identify the transmitting sender.
In a third exemplary embodiment (pulse only) of the invention, the ID code of each sender is defined by a unique sequence of quiet interval durations between marker pulses transmitted during each frame. The monitor includes a storage means storing a repertoire of sender ID codes, i.e., a set of interval durations for each sender. Additionally, the monitor includes a memory, e.g., a shift register, which maintains a history of recently received composite signal pulses. Each time a new pulse is received, the monitor compares the pulse pattern of each stored sender ID code against the pulse history looking for a match. When a match is recognized, the associated sender is confirmed as having xe2x80x9cchecked-inxe2x80x9d and a free running timer associated with that sender is reset. As long as the timer is reset before it counts to a threshold timeout value, the sender is deemed to be continuously present.
In a fourth exemplary embodiment (pulse/group synch) of the invention, the pulse pattern of the third embodiment is supplemented by a group synch (GS) pulse pattern common to all of the senders. Thus, the GS pattern defines the set of members and enables monitor to more rapidly identify the sender ID codes within the composite signal received from all senders.
In a fifth exemplary embodiment (group synch/symbol) of the invention, the fourth embodiment ID code pulse pattern is subdivided into multiple sub-codes. In this embodiment, each sub-code is able to communicate a different symbol to the monitor. Symbols, for example, might be numbers. That is, each symbol could be used to represent a number from 0 to 99. With an ID pattern defining three such symbols, the monitor can discern 1,000,000 possible unique sender-identifying ID codes. This embodiment finds service in applications where the population of the member set is large but relatively few members are present at any given time. An example of such an application is the monitoring and logging of warehouse items flowing down a conveyor belt for shipment.
Several variants of the aforementioned embodiments are also described hereinafter. For example, a correlation processing capability can be advantageously introduced into the several embodiments. This capability involves the process of correlating a received data stream with pulse patterns associated with known sender ID codes. The known sender ID codes are stored as binary patterns in the monitor""s repertoire memory. The received data stream originates as a graded signal, e.g., a radio baseband signal. This graded signal can be converted to digital format and processed arithmetically or the received data stream can be first processed by a thresholding element so that the data is converted to a binary pulse train format. In either case, correlation processing involves comparing two signals and computing a correlation score. The resulting scores are compared to discriminate threshold in order to make a decision as to the presence or absence of a particular sender. Correlation processing permits enhanced detection of sender pulse patterns when the incoming data is partially corrupted or incomplete.
In another variant, one or more data pulses can be positioned in an ID frame to enable a sender to communicate data, e.g., temperature or flow rate, to the monitor.